1. Field of the Invention
The present invention relates to a recording medium for information having a recording layer in which crystalline phases and amorphous phases are able to be formed alternately and reversibly at predetermined intervals by irradiating light beams such as laser beams with alternately changing irradiation conditions thereof, and from which information once recorded can be retrieved by utilizing the difference in intensity between light signals reflected at these different phases formed in the recording layer respectively.
2. Description of the Background Art
Conventionally, phase change type materials whose a light reflectance depends on solid phases are known as an optical disc material capable of storing information at a high density and erasing and writing information as desired.
Said phase change type material has a specific characteristic. Thus, portions in the material irradiated by the laser beams are reversibly changed between the different phases when laser beams are irradiated onto the material with changed irradiation conditions thereof, for example irradiation energy, irradiation intensity or irradiation pulse width of time.
For example, the phase change type materials having said characteristic include semiconductors such as tellurium (Te) and gerumanium (Ge), alloy TeGe, InSe which is an alloy of indium (In) and selenium (Se), SbSe consisting of antimony (Sb) and selenium (Se), SbTe consisting of antimony and tellurium, and intermetallic compounds.
When this type of material is irradiated by a laser beam, a crystalline phase or an amorphous phase is formed at the irradiation portion of the material in accordance with the irradiation condition. These two phases have different complex refractive indexes N=n (1-ik) (where n is a refractive index and k is an absorption index). Namely, since not only the refractive indexes n of these two phases but also the absorption indexes k thereof are different from each other, the values of light reflectance of these phases are also different from each other.
Thus, it has been proposed to use the material having such a characteristic as an optical memory capable of erasing and writing information by S. R. Ovshinsky et al. (Metallurgical Transactions 2 641 (1971).
According to this proposition, a disc which interposes a recording layer comprising the material as mentioned above is rotated, and a laser beam is irradiated onto the disc with alternately changed irradiation conditions thereof to reversibly form a predetermined pattern in which crystalline phases and amorphous phases are arranged alternately. Then, a laser beam for reading-out having much smaller irradiation energy is successively irradiated onto the crystalline phases and amorphous phases of the recording layer. The irradiated laser beam for reading-out is reflected by a reflecting layer optionally provided behind the recording layer after passing through its layer Thereafter, the reflected laser beam for reading-out is converted into a reading-out signal by a suitable photoelectric converter, and intensity of the laser beams passed through the different phases having specific light reflectance respectively are measured respectively. Namely, information showed by the phase pattern can be read out by discriminating the states of the phases by utilizing the difference in light reflectance.
In more detail, when some information is recorded in the recording layer formed with the material as mentioned above, a laser beam having a power high enough to heat the recording layer over the melting point thereof and having a short pulse width is irradiated onto the recording layer. Thus the recording layer is melted by the laser beam. Then, portions in the recording layer irradiated by the beam are quenched immediately to provide a recording mark being the amorphous phase at each of the portions. When information, that is, the phase pattern as mentioned above once recorded in the recording layer is erased, a laser beam having energy enough to heat the layer to a desired temperature exceeding the temperature for crystallization thereof and being lower than the melting point and having a relatively long pulse width is irradiated onto the recording layer. Then, portions in the recording layer irradiated by the beam are cooled down gradually to change all of the recording layer including the amorphous phases into a crystalline phase. Namely, the recording marks are erased.
In the manner of storing and erasing information, a so-called two beam method is adopted, in which a first laser beam having a circular spot for changing portions irradiated by the beam to the amorphous phases by melting and quenching and another laser beam having an elliptical spot for changing portions irradiated by the beam to the crystalline phase by cooling down gradually are used alternately and independently.
However, the two beam method requires a complicated optical system for irradiating these laser beams. Particularly, it is difficult to control the laser beam having an elliptical spot and a circular spot to follow a spiral track in a disc. In other words it is difficult to arrange those spots in a same track.
As a result, a so-called one beam method in which the storing and erasing of information are carried out by one laser beam has been studied.
In the one beam method, since a source of a laser beam which follows the track in the disc is single, the so-called overwrite, that is, erasing information already recorded in the recording layer and reading new information, can be easily carried out.
To be concrete, a power-changeable single laser beam source is used in the method, and a laser beam for erasing having a power level of Pe and another laser beam for recording having a power level of Pw (Pe&lt;Pw) are irradiated alternately at predetermined intervals onto the recording layer in which information is already recorded. As a result, new information is read in the recording layer and old information is erased.
The method for carrying out the overwrite by using a single laser beam source is called the one beam overwrite method, and does not require such a complicated servo system as required in the two beam method, thus the laser beam can be easily positioned.
However, the one beam overwrite method also has problems when it is adopted in the phase change type recording layer.
Namely, since the rotation speed of the disc is constant, the moving speed of the laser beam irradiated onto the recording layer to form amorphous phases or crystalline phases therein becomes constant. In other words, the irradiation time of the laser beam onto each specific portion of the recording layer is constant irrespectively of a kind of phase to be formed at the portion, and the conditions for discharging heat generated by the beam from the recording layer is the same irrespectively of a kind of phase to be formed therein. Therefore, whether a portion of the recording layer irradiated by the laser beam becomes the amorphous phase or crystalline phase depends only on the amount of power of the laser beam.
Accordingly, since it is necessary to carry out the crystallization in such a short time as required in forming the amorphous phases, it is difficult to complete the crystallization by taking enough time for cooling down for erasing information.
Moreover, when information is recorded, it is difficult to change the end portions of the recording mark to be provided into the amorphous phase. Namely, when the recording mark in the amorphous phase is provided, portions adjacent to the recording mark are heated up by the laser beam for erasing just before and immediately after the formation of the mark. Thus, the end portions of the recording mark can not be quenched immediately for the heat.
As a result, amorphous phases and crystalline phases can not be formed at predetermined intervals alternately. This causes wrong retrieval of information.
An alloy material comprising indium (In), antimony (Sb) and tellurium (Te) has been noted recently. That is, the phase change type recording layer consisting of the material allows the one beam overwrite method to be applied. Moreover, the application of the one beam overwrite method is enabled by selecting a composition of the alloy material suitably.
Namely, according to experiments done by the inventor of this invention, by using an alloy material formed by adding tellurium to a compound InSb by 20 to 45 atom %, in other words, an alloy material having a composition represented by In.sub.50-X/2 Sb.sub.50-X/2 Te.sub.X (20.ltoreq.X.ltoreq.45) comprising Te of X atom %, In of 50-X/2 atom %, and Sb of 50-X/2 atom % as a recording layer, a material having a rapid crystallizing characteristics of the intermetallic compound InSb and an easy amorphous characteristics of tellurium can be obtained. Namely, this alloy material can satisfy the necessity to carry out the crystallization in a short time and to form amorphous phases without quenching immediately, which are problems in the one beam overwrite method.
Moreover, since the alloy material changes to the amorphous state easily, it can change to the amorphous state even when the power of the laser beam for recording is considerably reduced. Therefore, it is possible to make the optical system for irradiating the laser beam in a small scale, thus an recording medium for information comprising the alloy material can be manufactured at low cost. Moreover, the recording medium for information can be handled easily.
However, since the recording layer having the composition represented by In.sub.50-X/2 Sb.sub.50-X/2 Te.sub.X is an alloy, the interatomic bonding power is weak and the composition is not so stable. Accordingly, when melting and quenching the recording layer are repeated in the overwrite operation, the composition of the alloy at portions subjected to the overwrite treatment changes gradually, that is, generates a segregation, and the rapid crystallizing characteristic and easy amorphous characteristic of the recording layer are deteriorated.
On the contrary, In.sub.3 SbTe.sub.2 which is a compositionally stable compound, is considered as a material useful as the recording layer in case that the one beam overwrite method is used.
Namely, a crystalline phase is formed extremely rapidly when a laser beam for erasing is irradiated onto In.sub.3 SbTe.sub.2 used as a material of the recording layer. This means that the compound In.sub.3 SbTe.sub.2 has the rapid crystallizing characteristic.
The reason is that the crystalline phase of In.sub.3 SbTe.sub.2 is extremely thermally stable.
Moreover, the composition of In.sub.3 SbTe.sub.2 is not changed by repetition of the overwrite operation since In.sub.3 SbTe.sub.2 is the compound. Accordingly, the rapid crystallizing characteristic is not deteriorated under the condition.
However, In.sub.3 SbTe.sub.2 is not likely to be changed to the amorphous state since the interatomic bonding power of In.sub.3 SbTe.sub.2 is high and the crystalline phase thereof is very stable.
Accordingly, it is necessary that the power of the laser beam for recording is much larger than that of the laser beam for erasing. The reason is that it is impossible to quench In.sub.3 SbTe.sub.2 immediately, that is, the amorphous state thereof can not be obtained since the circumference of a portion to be in the amorphous state is at a high temperature due to the a remaining heat of the laser beam for erasing, even if the temperature of In.sub.3 SbTe.sub.2 goes up to a melting point of In.sub.3 SbTe.sub.2 and then In.sub.3 SbTe.sub.2 is melted.
Accordingly, in an optical disc in which the compound In.sub.3 SbTe.sub.2 is used as the recording layer material, the laser beam for recording requires a very high power, that is, it requires a power of 22 to 25 mW at the surface of the disc, while the power of the laser beam required for erasing is at about 8 mW.
This means not only that the energy consumption becomes large but also that the optical system is required to be a large scale and extremely expensive. On addition, handling is very troublesome.